My invention relates to apparatus for sensing the presence or precise position of an object and, more particularly, to apparatus that performs such object sensing by measurement of capacitance.
In certain fields of engineering and industry, such as automation and robotics, it is important to detect presence and position of objects accurately for subsequent handling thereof. For example, detecting the location of a screw head may be necessary in order for a screw insertion device on an assembly line to engage the screw properly and efficiently and to fasten two or more parts to one another. Similarly, the placement of electrical components on a printed circuit board can be facilitated by accurately determining their initial position prior to their selection.
Often, due to the nature of the object being measured or the equipment subsequently used, a sensing tool that makes physical contact with the object is inappropriate. For example, relatively fragile objects should not be unduly manipulated or subjected to force. Likewise, the geometry of certain assembly apparatus or scientific instruments may not land themselves to contact measurement of objects. Finally, objects in motion would generally not be good candidates for contact measurement, as such contact would tend to impede such motion.
Non-contact measuring devices such as optical, electromagnetic, infrared, eddy current and electrostatic capacitance sensors, sometimes under control of computer programs, have been used with varying degrees of success. U.S. Pat. No. 4,686,531 issued to Shambroom et al discloses a capacitance height gage having four measuring and two reference capacitor circuits. Four measuring plates are disposed opposite the object surface to be measured and the object surface itself forms the ground plate. Reference capacitors provide voltage regulation to ensure that a stable signal drives the measuring capacitors.
U.S. Pat. No. 4,451,780 issued to Ogasawara discloses a non-contact capacitance microdisplacement meter having two different sized electrodees disposed opposite the object to be measured. Two electrostatic capacitances are thus generated and coupled to separate oscillator circuits to result in a displacement signal indicative of displacement of the object.
One common limitation of prior capacitative sensors is their susceptibility to error when they encounter objects of varying size. For many applications it is desirable that a capacitive sensor provide a signal when a probe tip being moved toward an object reaches a desired distance from the object, irrespective of the size thereof. To pose a simplified example, it might be desired to provide a signal when a movable probe tip reaches a distance of 0.05 inch from a coin. Prior capacitive sensors will readily provide such a signal when a probe tip is 0.05 inch from a given size coin, such as a dime; but such prior sensors tend to provide the same signal when the probe tip is a greater distance, say 0.10 inch, from a larger coin, such as a quarter. If the prior sensors are adjusted to provide the appropriate signal when the probe tip is the desired 0.05 inch from the quarter, they do not provide the signal until the probe tip is nearer than desired, say 0.02 inch, from the dime.
The aforementioned problem encountered by prior art devices makes it impossible to preset the position sensor with respect to the objects being detected unless all such objects are of identical size. In certain environments, that is simply not the case.
It might be desired that the signal be produced when a probe tip became 1.00 inch (rather than 0.05 inch) from a coin of any size, or at various other distances. Prior art measuring devices are not normally amenable to such adjustments or, when they are, not without necessitating significant effort and setup time on the part of the technician.
Thus a primary object of the present invention is to provide a capacitive sensor which will provide a signal when a probe tip reaches a desired distance from an object, irrespective, within limits, of the size or shape of the object.
Another object of the present invention is to make readily adjustable the distance at which the signal occurs, for an object of any shape and size, over a range of sizes.
Still another object of the invention is to provide apparatus for determining position of an object in a single sensing operation whether it is metallic or non-metallic.
Yet another object of the invention is to facilitate presetting of the distance between object and sensor prior to operation thereof so that a substantially constant distance between object and sensor can be maintained notwithstanding variations in object size.
Other objects of the invention will in part be obvious and will in part appear hereinafter.